1. Field of the Invention
This invention relates generally to a powertrain and brake system for a hybrid electric vehicle (HEV) and, more particularly, to its control during an engine restart.
2. Description of the Prior Art
A HEV is a vehicle that combines a conventional propulsion system, which includes an internal combustion engine and a transmission, a rechargeable energy storage system that includes an electric motor and electric storage battery to improve fuel economy over a conventional vehicle.
Motor vehicles can be designed to employ certain aspects of hybrid electric technology, but without use of a hybrid electric powertrain. Certain vehicles having a conventional powertrain but no electric machine for driving the wheels, called micro-HEVs, shutdown the engine at idle speed to reduce fuel consumption and reduce emissions while the vehicle is stopped.
During normal vehicle operation many instances arise where the vehicle must stop: at traffic signals, cross-walks, stop signs and the like. In micro-HEVs the engine is shut down if no power is required, e. g. while waiting at a traffic light. As soon as power is requested, the engine is automatically restarted. By avoiding an unnecessary engine idling event, the vehicle's fuel economy is improved. For this purpose, it is desirable to shut down the engine function as much as possible when certain engine stop conditions are satisfied.
An engine restart occurs while the transmission is in gear and the gear selector is in the Drive position. During an engine restart in a micro-HEV equipped with an automatic transmission and torque converter, a torque surge occurs substantially in phase with the engine startup speed peak.
During the time of engine automatic stop the intake manifold pressure has increased from the throttled vacuum of normal running condition to a higher pressure very close or equal to atmospheric pressure. At time of engine restart, this higher atmospheric pressure in the manifold leads to a very large air charge to the combustion cylinders. To achieve the emissions requirements, the proper engine out exhaust “feedgas” fed to the catalyst shall not have an abundance of either oxygen or incompletely combusted fuel by-products. In this respect, the large air charge at restart is matched by a proportionately large stoichiometric fuel injection mass, with the result being a large torque spike on the first several restart combustion events. After the manifold pressure is reduced to a throttled vacuum by nature of the mostly closed throttle and air being withdrawn by cylinders, all of the air charge, fuel injection, torque production, and engine speed decay back to usual idle conditions.
The torque spike, which is generated by the engine and amplified by the torque converter during the engine restart, is transmitted to the driven wheels if the engine is restarted with the transmission in-gear. The torque spike produces a forward acceleration pulse. Such an uncomfortable vehicle jerk adversely affects the driver's acceptance to the stop/start function.
A need exists for a technique that suppresses the torque surge during an engine restart event in a micro-HEV powertrain.